Siemens SIPROTEC 4 7SJ61 Manual page 84

Functions
2.4 Single-Phase Overcurrent Protection
[ueb-einph-widerstand-021026-rei, 1, en_US]
Calculation Example:
For the 5 A CT as above
desired pickup value Ι
[ueb-einph-widerstand-5a-021026-rei, 1, en_US]
For the 1 A CT as above
desired pickup valueΙ
[ueb-einph-widerstand-1a-021026-rei, 1, en_US]
Series resistor R must be rated for a minimum permanent load P
[fohochimppcont5a-20120514, 1, en_US]
[fohochimppcont1a-20120514, 1, en_US]
Furthermore, series resistor R must be rated for a fault current applying for approx. 0.5 s. This time is usually
sufficient for the backup protection to clear the fault.
The thermal load of the series resistor depends on the voltage V
calculated according to the following formulas:
[fohochimpvrmsstab5a-20120514, 1, en_US]
[fohochimpvrmsstab1a-20120514, 1, en_US]
where I corresponds to the maximum fault current during an internal fault.
k,max,int
For the 5 A current transformer 800/5 with 40 kA primary, Ι
For the 1 A current transformer 800/1 with 40 kA primary Ι
This yields a short-time load over 0.5 s for the series resistor of:
[fohochimpp-5a-20120514, 1, en_US]
84
= 0.1 A (equivalent to 16 A primary)
pu
= 0.05 A (equivalent to 40 A primary)
pu
:
permanent
applying during an internal fault. It is
rms,stab
equals 250 A secondary.
k,max,int
50 A secondary.
k,max,int
C53000-G1140-C210-6, Edition 05.2016
SIPROTEC 4, 7SJ61, Manual